In analog signal processing, a resistance element has been used widely as an element that converts a current signal to a voltage signal. In such a circuit, a resistance value of the resistance element affects circuit characteristics such as a gain of an amplifier circuit and a cutoff frequency of a filter circuit, for example. Generally, there is a certain range of variations in the resistance value of a resistance element formed in a semiconductor process, and the resistance value sometimes depends on an ambient temperature.
In order to keep the gain of an amplifier circuit of a resistance load constant, for example, it is necessary to keep the gain to a constant value by correcting variations or fluctuations in the resistance value of the resistance load. As one method of keeping the gain to a constant value by correcting variations or fluctuations in the resistance value, there is a method of using a variable resistance circuit capable of changing the resistance value by means of control.
FIG. 6A is a diagram illustrating a configuration example of a variable resistance circuit in which a resistance value is controlled in a digital manner. The variable resistance circuit includes a plurality of resistors 601-1, 601-2, . . . , 601-N, and PMOS transistors 602-1, 602-2, . . . , 602-N controlling whether or not to supply current are connected to the resistors 601-1, 601-2, . . . , 601-N respectively.
The PMOS transistors 602-1, 602-2, . . . , 602-N are controlled so as to be brought into an on state (conductive state)/an off state (non-conductive state) by control signals S1, S2, . . . , SN to be supplied to gates thereof respectively. The number of the PMOS transistors 602-1, 602-2, . . . , 602-N to be brought into an on state is controlled by the control signals S1, S2, . . . , SN, thereby making it possible to control the number of the resistors 601-1, 601-2, . . . , 601-N in which the current flows between a terminal and a power supply potential and change a combined resistance value.
FIG. 6B is a diagram illustrating a configuration example of a variable resistance circuit in which a resistance value is controlled in an analog manner. Resistors 611, 612 are connected in series between a power supply potential and a terminal, and a PMOS transistor 613 as a variable resistance is connected in parallel to the resistor 611. A gate voltage VG is supplied to a gate of the PMOS transistor 613, and by controlling the voltage VG, the PMOS transistor 613 controls on-resistance corresponding to a gate-source voltage.
When a resistance value of the resistor 611 is set to RP, a resistance value of the resistor 612 is set to RS, and a resistance value of the on-resistance of the PMOS transistor 613 is set to RON, in the configuration illustrated in FIG. 6B, a combined resistance value RS+{RP·RON/(RP+RON)} is obtained. For example, when the PMOS transistor 613 is in an on state (the resistance value RON of the on-resistance is almost zero), the combined resistance value becomes RS, and when the PMOS transistor 613 is in an off state, the combined resistance value becomes (RS+RP).
Accordingly, the variable resistance circuit illustrated in FIG. 6B can change the resistance value from the resistance value RS to the resistance value (RS+RP) by controlling the gate voltage VG to be supplied to the gate of the PMOS transistor 613. Incidentally, the resistor 611 illustrated in FIG. 6B can be omitted by bringing into an open state (infinite resistance), and the resistor 612 can be omitted by bringing into a short-circuit state (zero resistance).
Such a variable resistance circuit as illustrated in FIG. 6A or 6B is applied as the resistance load of the amplifier circuit to change the resistance value by means of control, thereby making it possible to keep the gain of the amplifier circuit constant. However, in the variable resistance circuit illustrated in FIG. 6A, an adjustment step of the resistance value is finite and the change in the resistance value is discrete, and thus accuracy with respect to a desired resistance value is limited. Further, in the case where control of the resistance values is performed by using control signals obtained by advance calibration or the like at the time of actual use, the variable resistance circuit does not follow fluctuations in the resistance value caused by environmental changes of temperature or the like after the calibration and a fluctuation amount results in an error.
Further, the variable resistance circuit illustrated in FIG. 6B controls a variable range of the resistance value by the gate voltage in a range where the PMOS transistor 613 is turned on, and thus the sensitivity of the gate voltage with respect to the resistance value is high. Therefore, when the variable resistance circuit tries to correspond to a resistance value in a wide range, a slight error of the gate voltage greatly affects the change in the resistance value. Accordingly, the resistance value of the variable resistance is liable to be affected by noise or the like.
Regarding the technique that corrects temperature-dependent properties, there has been proposed a voltage generation circuit that includes a circuit performing temperature compensation by analog control and a circuit performing temperature compensation by digital control and performs switching between the analog control and the digital control according to a temperature region (for example, Patent Literature 1). Further, there has been proposed a sensor amplification circuit that performs, at correction points set at predetermined temperature intervals, digital correction to correct an input signal to a target value based on correction data set beforehand for each correction point and performs analog correction to offset temperature dependency of the input signal based on a gradient calculated from the correction points that are adjacent to each other between the correction points (see Patent Literature 2, for example).
Patent Literature 1: Japanese Laid-open Patent Publication No. 2003-84728
Patent Literature 2: Japanese Laid-open Patent Publication No. 2007-248288